1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device using a liquid crystal display cell which comprises a pair of opposed substrates disposed with liquid crystal interposed therebetween and a resistance layer provided on that surface of one of the substrates which is in contact with the liquid crystal.
2. Description of the Prior Art
In order to convert the magnitude of a voltage applied into a position of a display surface, devices have heretofore been considered in which the voltage applied is analog-digitally converted, for example, and the digital signal is applied to a group of divided electrodes, whereas such devices have suffered from complexity of the circuit arrangement. To overcome such disadvantage, various means whereby analog signals may directly be analog-displayed have also been proposed in the display devices using liquid crystal.
In some of these devices, with the fact taken into account that the magnitude of the electric field between opposed electrodes is a function of the distance therebetween, the electrodes are opposed at non-uniform interval so that the operative location of the liquid crystal is varied by the magnitude of the applied voltage, thereby effecting the display.
There is another form of the liquid crystal display device in which one of two opposed electrodes of the liquid crystal display cell is divided into several electrode members and functional elements having different threshold voltages are coupled to these electrode members. This display device is such that when a voltage is applied to a common external connection terminal of the function elements, one of the functional elements whose threshold voltage is exceeded by the applied voltage is rendered operative and the liquid crystal display cell connected to that functional element is turned on so that the position display of the voltage is directly effected.
There is a further form of the display device in which liquid crystal display cells are connected together by a resistor so that the voltage applied to the liquid crystal display cells is varied by the resistor to thereby effect the display.
Among the various forms of liquid crystal display device mentioned above, the display device which effects position display in accordance with the magnitude of the applied voltage, by the use of a liquid crystal display cell comprising two electrodes opposed at non-uniform intervals positionally displays the magnitude of the applied voltage in accordance with the delay in response of the liquid crystal. The delay results from the different intensities of field imparted to different portions of the liquid crystal layer intervening between the two electrodes opposed to each other at non-uniform interval. Therefore, when a voltage exceeding the threshold voltage is applied, the display position varies depending on time and it is thus difficult to obtain reproducibility of the display positions. Further, if a voltage is applied for a time exceeding the aforementioned response time, the liquid crystal always throughout the entire display area of the liquid crystal display cell will become operative to effect only ON-OFF display, so that position display in accordance with the magnitude of the voltage will be impossible.
Also, these devices offer other various problems such as difficulties in regularizing the manufacturing conditions thereof, the high dependency on temperature and therefore, the necessity of eliminating the influence of the ambient temperature during operation.
Among the above-described liquid crystal display devices, those which do not utilize the intrinsic characteristic of the liquid crystal, but employ different functional elements connected to the liquid crystal, offer no essential problem in principle. Nevertheless, they are complicated and expensive to use and cannot be said to be commercially excellent.
In contrast, a display device based on the concept of imparting different voltage distributions to the liquid crystal layer from position to position thereof is theoretically possible and permits structural simplification. Therefore, realization of such display device is most eagerly desired. In fact, in an effort to realize such display device, several inventions have been proposed including that disclosed in Japanese Patent Publication No. 3642/1971 which uses a liquid crystal display cell utilizing the dynamic scattering mode (DSM) of the liquid crystal. In this device, a voltage distribution is imparted to one of two electrodes and a control voltage is imparted to the other electrode, whereby the position whereat the voltage is equal to the threshold voltage defines a boundary which divides the display surface into the under-threshold voltage area and the over-threshold voltage area, so that analog display is effected in the form of bar-like position display by a transparent portion and an emulsified portion.
As a further development of such invention, there has been proposed a device in which a number of bar-like displays similar to the aforesaid bar-like display are arrayed and voltage signals corresponding to changes of time are imparted to respective ones of the bar-like displays to effect oscillographic display (see Japanese Patent Publication No. 7377/1971), and an improvement over this device is the display device disclosed in Japanese Patent Publication No. 7391/1971.
A further device has also been proposed in which a film-like resistor is connected to divided electrodes to cause a similar effect.
However, these devices still offer various problems as will hereinafter be described, and have not yet been put into practice.
Firstly, the display by a liquid crystal display cell utilizing the DSM is effected by the change of condition between transparency and emulsification and cannot perform the essential color display. Secondly, the change of condition between transparency and emulsification cannot provide a sufficiently high optical contrast. Thirdly, the threshold characteristic of the liquid crystal display cell utilizing the DSM is originally slow and the boundary area of the display is unclear. Further, if a dope such as an electrolyte or the like is added to he liquid crystal to improve this, more current will flow through the liquid crystal layer to limit the performance and reduce the life of the cell.
Furthermore, in the display device which tries to make clear the boundary area by the use of divided electrodes, there is a disadvantage on the one hand that the display becomes a stepwise display instead of a continuous position change display and, thus, the display is not free of the limitations in steps of display. On the other hand, in the device using electrodes subdivided from the divided electrodes in an effort to effect an apparently continuous display, there is a disadvantage which is essentially attributable to the problem of unclear boundary area. In addition, machining of subdivided electrodes involves a high degree of technique in the manufacture and leads to an increased number of manufacturing steps which is commercially inconvenient. Also, even if the display method disclosed in the aforementioned Japanese Patent Publication No. 3642/1971 is utilized and a field effect mode (FEM) liquid crystal display cell is simply employed in place of the DSM liquid crystal display cell, the unclear display of the boundary area is essentially unavoidable. This is rather similar to the invention disclosed in Japanese Patent Publication No. 98599/1974 in that these are intended for the purpose of effecting tone display of color and brightness.
A further point in which the prior art methods including these are to be improved is the dependency of the threshold on temperature. In other words, for the same voltage applied, the boundary position is displaceable by temperature and this is conspicuous in the methods of the prior art. Thus, some auxiliary means for carrying out temperature compensation or the like becomes necessary to avoid such an effect and this in turn may result in complication of the device and accordingly, an increased number of manufacturing steps and greater cost of manufacture.
From another point of view, it has been very much desired to exploit a display device of simple circuit construction in which a plurality of different information signals may be applied as inputs to a display cell to provide a plurality of display outputs or to display a plurality of information signals and in which coincidence or non-coincidence between the plurality of information signals may be discriminated on the display surface.
However, if a display device having the above-described performance could be provided by the use of an ammeter or LED display cell which is the best-known display cell, the inertia of the movable portion of the ammeter would cause a slow response and when a plurality of different signals were alternately applied, the movable portion would repeat vibration about the average value of two inputs with the result that it would be impossible for a single display device to display two or more different display outputs apparently at a time. If the so-called 7-segment LED display cell were used to effect numerical display, the driving circuit therefor would be complex and in addition, a numerical display changing at short time intervals could not be read. Alternatively, the use of a dotted LED display cell would necessitate an AD converter circuit for converting analog signal inputs into digital signals and this would not only lead to complication of the circuit arrangement but also result in a stepwise display instead of a continuous display.